Polybutylene terephthalate (hereinafter sometimes abbreviated as “PBT”), which is a typical engineering plastic among thermoplastic polyester resins, is excellent in view of the easy molding process, mechanical property, heat resistance, chemical resistance, aroma retention property, and other physical and chemical properties. Thus, PBT has been widely used as a material for injection-molded parts such as automotive parts, electric and electronic parts, and parts for a precision instrument. Recently, as PBT has been widely used also in the field of general consumer goods such as a film, a sheet, a monofilament and fibers making use of its excellent properties, PBT with an excellent color has been required.
PBT can be generally obtained by reacting terephthalic acid or an alkyl ester thereof with 1,4-butanediol (hereinafter sometimes abbreviated as “1,4-BG”). However, since 1,4-BG is easily converted into tetrahydrofuran (hereinafter sometimes abbreviated as “THF”) during the reaction, a method for producing PBT in high yield in which the conversion of 1,4-BG into THF is prevented is required.
In addition, with the growing demand for building a sustainable society, it is desired to switch from materials derived from fossil fuel in the production of PBT as in the energy production. When no fossil fuel is used as a raw material, a biomass-derived material obtained from plants and the like as raw materials is one of the possible raw materials. Also with regard to 1,4-BG, which is a raw material of PBT, a method for producing PBT from biomass-derived 1,4-BG as a raw material has been studied.
However, the color of obtained PBT deteriorates when biomass-derived raw material 1,4-BG is used, in comparison with the color of PBT produced from raw material 1,4-BG which is obtained from fossil fuel such as petroleum. The major cause for the color deterioration is thought to be a nitrogen-containing component in PBT. In addition, it is thought that components other than this component also have an influence on the color of PBT.
PTL 1 describes a technique for obtaining PBT from a biomass-derived raw material and describes that PBT with a nitrogen content of 50 ppm by mass or less is obtained by setting the nitrogen content in the biomass-derived raw material 1,4-BG within 0.01 to 50 ppm by mass. PTL 1 also describes that, although 1-acetoxy-4-hydroxybutane (hereinafter sometimes abbreviated as “1,4-HAB”) contained in the raw material 1,4-BG delays the polycondensation reaction of PBT and causes the coloration of PBT, the coloration of PBT due to delayed polymerization can be reduced by using, as a raw material, 1,4-BG obtained from raw material 1,4-BG with a controlled nitrogen concentration.
In addition, it is known that gamma butyrolactone (hereinafter sometimes abbreviated as “GBL”) is generated as a by-product in the conventional methods for producing 1,4-BG. For example, PTL 2 describes that gamma butyrolactone is produced as a by-product when a crude hydrogenated product containing 1,4-BG is obtained by hydrogenating maleic acid, succinic acid, maleic anhydride, fumaric acid and/or the like, in a method for producing 1,4-BG from a raw material derived from fossil fuel.
As methods for producing 1,4-BG from a biomass-derived raw material, PTL 3 describes to subject biomass-derived succinic acid to chemical reduction or bioengineering hydrogenation to produce 1,4-BG, and PTL 4 describes a method for obtaining 1,4-BG by direct fermentation from bacterial cells.